LoopGenerators: Construct loops such that they are already loop rotated

                BeforeBB
                   |
                   v
                GuardBB
                /      \
       __  PreHeaderBB  \
      /  \    /         |
   latch  HeaderBB      |
      \  /    \         /
       <       \       /
                \     /
                ExitBB

This does not only remove the need for an explicit loop rotate pass, but it also
gives us the possibility to skip the construction of the guard condition in case
the loop is known to be executed at least once. We do not yet exploit this, but
by implementing this analysis in the isl code generator we should be able to
remove more guards than the generic loop rotate pass can.  Another point is that
loop rotation can introduce additional PHI nodes, which may hide that a loop can
be executed in parallel. This change avoids this complication and will make it
easier to move the openmp code generation into a separate pass.

llvm-svn: 181986